Some industrial processes call for the introduction of fluids into rotating tubes. In many such situations it is desirable, if not essential, that this be done without ambient matter becoming entrained with the fluid as it flows from a stationary conduit into the rotating tube. For example, in constructing preforms from which optical waveguide fibers may be drawn, vapors of materials such as SiCl.sub.4, GeCl.sub.4, BCl.sub.3 and POCl.sub.3 are entrained in an oxidizing carrier gas such as oxygen. The vapor stream is then drawn through a stationary conduit and into a rotating glass preform starter tube. In order to inhibit the vapor stream from leaking to ambient atmosphere, and ambient air and airbourne inpurities from entering and thereby contaminating the vapor stream, a sealed rotary joint has been provided at the junction of the stationary and rotary tubes. This joint has been provided by locating an end portion of one of the tubes within an end portion of the other tube and positioning one or more resilient O-rings or washers between the two tubes. This arrangement, however, has been less than satisfactory since at least one of the tubes is constantly rubbing against the resilient O-rings causing them to become heated and to wear out. Structural deteriorations of the O-rings soon leads to leakage which is aggravated whenever, as here, there is a pressure differential between the fluid stream and ambient atmosphere. Furthermore, in such highly controlled situations as optical waveguide fiber preform manufacture even a very slight leak can create severe problems. For example, a PPM leak to ambient surroundings can endanger personnel since the vapor stream is highly toxic. Such leakage will also alter the rate of vapor stream flow into the preform which rate much be precisely controlled and maintained. Conversely, an ingress of ambient air will also alter the flow rate as well as contaminate the vapor stream with water vapor and airbourne impurities.
As shown in U.S. Pat. No. 347,069 which is assigned to the assignee of the present invention and application, improvements have recently been made in the art of introducing a vapor stream into a rotating optical fiber preform tube. The method disclosed in that application comprises the steps of generating a vapor stream comprised of a vaporized gas-forming precursor entrained in an oxidizing carrier gas, and flowing the vapor stream into the rotating optical fiber preform tube through an at least partially sealed rotary joint. Any material alteration in the composition of the vapor stream is prevented from occurring should the rotary joint seal become leaky by flowing a stream of fluid consisting essentially of the same oxidizing carrier gas as that in the vapor stream over and about the rotary joint at a pressure greater than the pressure of the vapor stream flowing through the rotary joint. With this method any ingress of ambient fluid into the preform tube at the rotary joint is in the form of a component of a vapor stream itself passing therethrough and a very small percentage of it at that. In this manner the qualitative composition of the vapor stream is maintained.
In the just mentioned application apparatus is also disclosed for supplying fluid to the rotary tube which includes an end cap having an open ended bore in which an end portion of a tube is rotatably positioned. Conduit means extend into the end cap through which fluid may be fed into the rotary tube while other conduit means communicate with the end cap bore through which a purge fluid may be fed into and at least partially through the bore to the exterior of the end cap.
Though the lastly described development has provided distinct advantages there nevertheless have remained certain problems associated with its use. For example, the end of the tubular extension of the preform has had to be rotatably positioned within a cylindrical cavity of the end cap with but a very small clearance present between the stationary end cap bore wall and the rotating tubular extension periphery. Too large a clearance increases the danger of ambient air ingressing into the housing and then into the tube. Conversely, too small of a clearance increases the danger of binding occurring between the two joint members. This is particularly true due to the temperature cycling associated with optical fiber preform manufacture and the heat conducted by the tubular extension from the preform that is being selectively heated to very high temperatures. Furthermore, since the optical fiber preform is composed of glass it often is out of round and/or bowed whereby its rotation causes the tubular extension thereof to wobble somewhat as it itself rotates along with the preform within the end cap. Such wobbling further increases the difficulty of maintaining a proper clearance between the rotating and stationary members of the rotary joint and increases the danger of binding.
Other prior art methods of sealing rotary joints in other technical applications have included the use of magnetizable fluids and ball bearings. For example, in U.S. Pat. No. 2,557,140 a magnetizable fluid is provided in a chamber disposed between two hollow members of the joint. Means also provided for magnetizing the fluid in the chamber to solidify it and thereby seal the joint. Such an approach though cannot be used in environments where any ingress of fluid into the joint from about the seal is in any form other than that of a constituent of the fluid being conveyed through the joint. U.S. Pat. No. 3,889,980 discloses another rotary joint which has a conduit with its inner end mounted within a housing in fluid communication with a housing passageway. Here, ball bearings are mounted in the housing surrounding the conduit and sealing means provided in the form of two sealing elements have opposing, mutually contacting seal faces which prevents fluid from reaching the ball bearings. The use of contacting faces, however, leads to friction and wear over a relatively brief period of time thereby limiting the effectiveness of such seals.
It thus is seen to remain a need for providing effective methods and apparatuses for supplying fluids to rotating tubes as is done in the manufacture of optical waveguide fiber preforms. It is to this task to which the present invention is primarily directed.